1. Field of the Invention
The present invention relates to frequency hopping spread spectrum techniques for reducing interference to narrowband receivers caused by baseband transmitters. More particularly, the present invention relates to frequency hopping spread spectrum techniques which slightly modify the pulse repetition rate generated by a baseband transmitter, thereby reducing interference to narrowband receivers.
2. The Prior Art
Spread-spectrum signals for digital communications were originally developed and used for military communications either to provide resistance to jamming or to hide the signal by transmitting it at low power and, thus, make it difficult for an unintended listener to detect its presence in noise. More recently, spread-spectrum signals have been used to provide reliable communications in a variety of civilian applications, including mobile vehicular communications.
There are several types of spread spectrum signals. In one type, the basic elements of a spread spectrum digital communication system include a channel encoder, modulator, channel decoder, demodulator, and two synchronized sequence generators, one which interfaces with the modulator at the transmitting end and the second which interfaces with a demodulator at the receiving end. These two generators produce a binary-valued sequence that is used to periodically change the carrier frequency and thus spread the transmitted signal frequency at the modulator and to follow the carrier frequency of the received signals at the demodulator.
In carrier-based frequency-hopped spread spectrum the available channel bandwidth is subdivided into a large number of non-overlapping frequency slots. In any signaling interval the transmitted signal carrier occupies one of the available frequency slots. The selection of the frequency slots in each signal interval is made either sequentially or pseudorandomly according to the output from a pseudo-noise generator. The receiver tuning follows the frequency hopping of the transmitted carrier.
Additionally, baseband wireless systems, also knows as ultra wideband (UWB) are well-known in the art of digital communications. These baseband wireless systems have the potential to produce interference to narrowband receivers, when one of the signal""s frequency harmonics falls within the narrowband receiver bandwidth.
Neither of these approaches teach an apparatus or method using frequency hopping spread spectrum technology to reduce the amount of interference produced by a baseband transmitter to narrowband receivers.
Accordingly it is an object of the invention to provide an apparatus and a method to reduce the amount of interference to narrowband receivers produced by a baseband transmitter.
It is a further object to provide an apparatus and a method for reducing the interference to narrowband receivers by slightly modifying the pulse repetition rate generated by a baseband transmitter, so that transmitted signals which fall within the frequency range of the narrowband receiver appear intermittently.
Other objects, together with the foregoing are attained in the exercise of the invention in the following description and resulting the embodiment illustrated in the accompanying drawings.
This present invention is a baseband frequency hopping apparatus and method which reduces the amount of interference generated by a baseband transmitter to narrowband receivers. In the present invention, the baseband transmitter may operate as a wireless transmitter or as a xe2x80x9cwiredxe2x80x9d transmitter which uses electrical cable, optical cable, or other cabling techniques to connect the baseband transmitter to a receiver. To accomplish this objective the invention periodically modifies the pulse repetition rate of the baseband transmitter. Transmitted signals which fall within the frequency range of the narrowband receiver appear intermittently, thus reducing interference to the narrowband receiver.
The present invention comprises a baseband transmitter configured to transmit a plurality of pulses with varying pulse repetition rates. The duration of the transmission (TT) is divisible into time periods (T) (also referred to herein as xe2x80x9cdata framesxe2x80x9d). Each time period (T) is further divisible into time slots (ts) (also referred to herein as xe2x80x9cdata slotsxe2x80x9d) which each have a specific pulse repetition frequency (f). The pulse repetition frequency (f) is the frequency at which the pulses are transmitted by the transmitter. The pulse repetition frequency changes for each time slot. The pulse repetition frequency may change at a constant rate or at a variable rate.
The present invention includes a synchronization sequence which selects a synchronization time slot and its corresponding pulse repetition frequency to generate signals for synchronizing the baseband transmitter to a baseband spread spectrum receiver.
In operation, the transmitter generates signals at a particular pulse repetition frequency. The pulse repetition rate of these transmitted pulses may fall within the receiving bandwidth of one or more narrowband receivers and may result in interference to narrowband receivers. To avoid such interference, the pulse repetition frequency of the baseband transmitter is offset for each time slot, either by a fixed frequency offset or by another manner.
The frequency offset reduces interference to narrowband receivers by varying the time between adjacent pulses such that the pulse repetition rate does not fall within the passband of a tuned narrowband receiver for any appreciable time.
In the present invention, frequency hopping is achieved by changing the pulse repetition frequency for each time slot. Thus the change in time slot varies the pulse repetition frequency and the duty cycle. Note that if the duty cycle is less than 100%, then the transmitter signals appear as bursts.
An additional benefit is derived if the time slot is less than the receiver""s filter time constant. If the time slot is smaller than the filter time constant of the receiver, the peak and average power communicated to the receiver is significantly reduced.